This invention relates to the manufacture of carbon black, and particularly to the types of carbon black used in compounding rubber, such as, for example, rubber for automobile tires. It relates specifically to the furnace process for making carbon black. It is concerned with the use of a unique combination of the component elements of gas turbine technology and carbon black technology.
For several decades in the early history of the automobile industry, the carbon black for compounding rubber for tires was furnished largely by the channel black process, in which gas flames a few inches high deposited carbon on channel irons in the presence of a deficient air supply. For economic reasons, the channel process has been almost wholly replaced by variants of the furnace process.
The apparatus of the furnace type process usually consists of a source of free-oxygen-containing gas, such as air from a blower, a combustion zone, a reaction zone, a quench zone, and a collecting system. The hot gases produced by the combustion of fuel in the combustion zone are confined in a flow through a reaction vessel, usually a steel shell lined with refractory, and ordinarily of circular cross-section. A feedstock hydrocarbon, usually an oil, which is the chief source of product carbon in the system, is injected into the flowing hot gases downstream of a point where the combustion of fuel is essentially complete. As the oil enters the flowing hot gases, it evaporates, and goes through pyrolysis, a series of thermal cracking and chemical reaction process steps in the reactor which produce carbon black in the reaction zone. The hot gases containing the carbon black are then quenched with water spray in the quench zone to a temperature low enough to stop the reactions, and to allow the carbon to be collected in a conventional collecting system.
There are two general categories of furnace carbon blacks used in the automotive tire industry: the so-called soft blacks used, largely in tire carcasses; and the blacks which impart high abrasion resistance to rubber used in tire thread, commonly known as "tread blacks". The tread blacks are much finer than the carcass blacks; that is, the particles are much smaller, and they have as a result, more surface area per unit of mass. The two types of carbon blacks are made under different reaction conditions, with the tread grade blacks being made at higher velocities (necessitating higher initial pressures), higher temperatures, and lower ratios of hydrocarbon feedstock to hot flowing gases than the carcass blacks.
In the conventional furnace process, the hot gases used for pyrolysis of the feedstock oil are produced by burning a hydrocarbon fuel, usually natural gas, in a stream of process air furnished by a blower. The hot gases from this combustion are at temperatures suitable for the cracking of a feedstock hydrocarbon to carbon black, generally 2000.degree. to 3400.degree. F. The feedstock hydrocarbon is then injected by suitable means into the stream of hot gases, which entrain the feedstock during its conversion to carbon black, and the carbon black after it is formed.
In conventional carbon black manufacturing processes, the process air may be preheated prior to combustion of the hydrocarbon fuel by indirect heat exchange with the hot, carbon-containing gases leaving the reaction zone. Such preheating of process air is especially useful in the manufacture of tread grade blacks, which require higher temperatures, since it raises the temperature of the combustion products while requiring the burning of less fuel. However, if an air to fuel ratio a little above stoichimetric is used, preheating the air may cause such high temperatures as will damage the refractory, or even melt it. The higher the temperature to which the air is preheated, the higher the air to fuel ratio must be to prevent overheating; that is, the flame must be more lean with respect to fuel. Thus, raising the air preheat temperature leads to increased free oxygen in the products of combustion. In cases of high air preheat to about 1000.degree. F, the amount of air may be as high as twice the stoichiometric quantity. The result is a significant portion of the feedstock is burned, rather than converted to carbon black. The first increments of air preheat above ambient temperature have been reported to increase the yield of carbon black based on feedstock, by speeding up the carbonization reeaction which competes with the oxidation reactions. At a point, however, depending upon the character of the feedstock, the process parameters, and residence time, the yield based on feedstock begins to decline. It is, then, desirable for the combustion zone of the reactor to produce combustion gases at the highest temperature which the refractory can continuously withstand, but with limited free oxygen.
One method commercially available for producing an oxygen containing gas with an oxygen content less than that of air, and from which a portion of the heat released by consumption of a part of the oxygen has been removed and put to a useful purpose, is to use the tail gases from a gas turbine or "turbofan" engine as process air for the reactor. The basic components of a gas turbine system are a combustor, a compressor, and a turbine. The compressor supplies air under pressure to the combustor where ordinarily a liquid fuel is burned, and from which hot gases flow at high velocity, with high energy content, into the turbine. The gases produce a rotary motion of the turbine as a result of being deflected by rings of blading on the rotor. The heat content of the gases is reduced by an amount proportional to the work done in rotating the turbine, and their temperature is correspondingly reduced. The turbine then drives the compressor which supplies air to the combustor. The combustion in the combustor must operate lean in fuel, i.e., at a ratio of air to fuel much higher than stoichiometric, to avoid overheating and damaging the blading of the turbine. As a result of these conditions, the exhaust gases from the turbine have a free oxygen content, but less than air; and are preheated, but a portion of the heat released by the combustor has been removed by doing the useful work of compressing air for the combustor. The turbine may, in addition, do other work, such as driving an electric generator, or furnishing direct motive power.